The T-Mobile contract states that its bills may increase in step with the Retail Price Index, a government-calculated rate of inflation. When this figure reached 3.3 per cent, T-Mobile and Orange - both run by mobile overlord EE - raised their prices accordingly.

But by the time letters went out informing subscribers of the change, RPI had dropped to 3.2 per cent, prompting internet clever-clogs to consider themselves released from their contractual obligations.

"We used the 3.3 per cent figure in anticipation of the issue of the RPI figure by the [Office for National Statistics]", T-Mobile told us. "Our terms and conditions do not permit customers to end their contract early in these circumstances."

The company's subscribers disagree, citing a purported extract from T-Mobile's own terms and conditions which supposedly allows instant termination of contracts if charges increase at a rate above RPI.

Anyone with a subsidised handset (i.e. most people) could potentially find themselves excused from having to complete the full term of their contract, as happened in 2007 when O2 stopped including non-geographic numbers in bundled call minutes.

Ofcom, the telcom regulator, is looking at the whole subject of intra-contract price changes. They are spelled out in the small print but still catch customers by surprise, so will probably disappear within the next few years.

Fixing a price for the length of a contract will make 24-month deals more risky for the operator, so probably more expensive for the customer, but reducing the length will be no bad thing - even if it slows down sales of smartphones. ®